With a growing world population, increasing demand for food, fuel and fiber, and a changing climate, agriculture faces unprecedented challenges. Development of plants with improved traits is highly desirable, with some of the major traits that are of major interest to farmers and seed companies include improved abiotic stress tolerance, fertilizer use efficiency, disease resistance, yield and more.
Plant trait improvement is typically performed by either genetic engineering or classical breeding. New methods for trait improvement through specific gene alteration are highly desirable. These include methods for over-expression of genes or gene silencing. A powerful technique for sequence-specific gene silencing is through RNA interference (RNAi). First discovered in the nematode C. elegans (Fire et al. 1998, Nature, 391:806-811), RNAi is a mechanism in which expression of an individual gene can be specifically silenced by introducing a double-stranded RNA (dsRNA) that is homologous to the selected gene into cells. Inside the cell, dsRNA molecules are cut into shorter fragments of 21-27 nucleotides by an RNase III-related enzyme (Dicer). These fragments, called small interfering RNAs (siRNAs), get incorporated into the RNA-induced silencing complex (RISC). After additional processing, the siRNAs are transformed into single-stranded RNAs that act as guide sequences to eventually cleave target messenger RNAs. By using RNAi to specifically silence relevant target genes, one can alter basic traits of an organism. Specifically for plants, it holds incredible potential for modifications that may lead to increased stress resistance and better crop yield.
In plants, RNAi is typically performed by producing transgenic plants that over-express a DNA fragment that is transcribed to produce a dsRNA. This dsRNA is then processed into siRNAs that mediate the cleavage and silencing of target genes.
The major technical limitation for this technology is that many important plant crop species are difficult or impossible to transform, precluding the constitutive expression of constructs directing production of dsRNA. Moreover, questions concerning the potential ecological impact of virus-resistant transgenic plants have so far significantly limited their use (Tepfer, 2002, Annu Rev. Phytopathol. 40, 467-491). An additional hurdle for obtaining transgenic plants is attributed to the difficulty of having the transformation and regeneration events occur in the same cell types.
Therefore the development of a method for obtaining transformed seeds which is independent of the methods inherent to tissue culture procedures is at the cutting edge of plant molecular biology research.